Update ngsi_arcgis_featuretable_sink.md

doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_arcgis_featuretable_sink.md

@@ -42,7 +42,7 @@ Each entity type needs an url and an unique field to be persisted into the featu
 
 NGSIArcgisFeatureTableSink composes each table's url with entitie's `service` and `service path`, to provide multiple tables access. 
 
-Unique field is provided to allow `NGSIArcgisFeatureTableSink` to update existant entities. NGSI `entity type` will be used as unique field name.
+Unique field is provided to allow `NGSIArcgisFeatureTableSink` to update existant entities. NGSI `entity type` will be used as unique field name. This means that a feature named `type` in the Feature Table cannot be filled in by the sink. If Feature Table needs to persist the value of entity type it has to be in a feature different than `type`.
 
 All this parameters, can be customized using Cygnus mapping capabilities.